1. Field of the Invention
This invention relates to single-phase, electronically commutated electrical machines. The invention is particularly, though not exclusively, applicable to single-phase switched reluctance motors.
2. Description of Related Art
Switched reluctance machines typically comprise a stator, with salient stator poles and stator windings for energizing the stator poles, and a rotor with salient rotor poles. The rotor is mounted in bearings to allow it to rotate coaxially with the stator. Movement of the rotor is influenced by the stator poles according to the energization of the stator windings. The theory and design of such machines is well documented in, for example, the paper `The Characteristics, Design and Applications of Switched Reluctance Motors and Drives` by Stephenson and Blake, presented at the PCIM '93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993, which is incorporated herein by reference.
In common with other types of electrical machines, switched reluctance machines can have one, two or more independent circuits, known as phases. These are discussed in the reference above. While each phase number is associated with a variety of different advantages, it is generally accepted that the single phase system is particularly cost-effective for low-power drive systems. An example of a single-phase machine is illustrated in FIG. 1. The stator S and the rotor R of the machine in FIG. 1 each have two salient poles, both stator poles having an exciting coil C wound around them. The two coils are connected together to form the phase winding. This example shows two poles on each of the stator and rotor, but other numbers of poles can be used. Further, this example shows the rotor rotating internally in the stator bore, though arrangements are known where the rotor rotates around the outside of the stator (these are known as `inverted` machines).
FIG. 2 illustrates the static torque curve of the machine, i.e. the torque T developed when an arbitrary, constant excitation current is applied to the phase winding as a function of the angle of rotation .theta. of the rotor. Such a curve is a characteristic of doubly salient motors and can be calculated or measured.
Machine control includes the `chopping` mode of motor torque regulation at low speed. The torque is controlled by inhibiting the winding current from rising above a maximum level by repeatedly chopping it during the phase conduction period. At higher speeds the rise and fall times for the current will be such that the current is switched on and off only once for each phase conduction period and is not chopped in normal operation. The torque is controlled through the switching angles, i.e. the angular positions at which voltage is applied to and reversed at the phase winding. This is the so-called `single-pulse mode` of operation. Both the chopping and single-pulse modes are discussed in the Stephenson and Blake paper referred to above.
Such machines are normally `rotor position switched`, i.e. the voltage is applied to and reversed at the phase winding according to the angular position of the rotor. The rotor position is determined either by a physical transducer which measures rotor shaft position or by algorithms which deduce the position from other variables in the machine, such as current and flux. In low-cost drives, simple position transducers are generally used, typically comprising an optical or Hall-effect device fixed to the stator and acting as a sensor to detect the proximity of a castellated vane or magnet assembly mounted on the rotor shaft. The vane usually has the same number of castellations as the rotor has poles, though other arrangements are possible.
Known arrangements of transducers typically use a single sensor for one-and two-phase systems, two sensors for a four-phase system and three sensors for a three-phase system. In a single-phase system with one sensor, simple logic is used to decode the output of the sensor to provide rotor position information within one rotor pole pitch. In the machine of FIG. 1, the sensor would normally be positioned to indicate when the rotor poles were fully aligned with the stator poles (i.e. the rotor position shown and generally known as L.sub.max, since the phase winding has maximum inductance at this rotor position). Assuming the vane has a mark/space ratio of unity, the sensor would also indicate the position 90.degree. away (generally known as L.sub.min, since the phase winding has minimum inductance at this position), i.e. with the rotor poles midway between the stator poles.
Positive torque is here defined as driving the motor in the forward, clockwise direction so that the angle of rotation .theta. is increasing positively in FIG. 2. It will be appreciated with reference to FIG. 2 that the motor will only start from rest in the forward direction if the motor is in a position corresponding to a region of positive torque, i.e. in the region between points 2 and 4 of FIG. 2. On the other hand, if the phase winding is energized when the rotor is in a region corresponding to negative torque, i.e. between points 5 and 6, the rotor will move in the reverse direction. If the phase winding is energized in a region of substantially zero torque, i.e. at least between points 1 and 2 or between points 4 and 5, the motor will fail to start.
In order to overcome this problem of unreliability in starting, various methods are known for positioning the rotor in the correct angular position with respect to the stator before energizing the phase winding, thus ensuring that the motor will start and will rotate in the desired direction. Such a system is disclosed in, for example, U.S. Pat. No. 4,932,069, incorporated by reference herein, which discloses the use of so-called `parking` magnets generally mounted on the stator and acting on the rotor to hold it at rest in a preferred starting position. Such a position would typically be around point 3 on FIG. 2, where the torque is at or near a maximum value.
There is one characteristic of parking magnets that is often apparent when the drive has been operating and is switched off. The rotor coasts down in speed until the speed is low enough for the magnets to `grab` the rotor and prevent further forward movement. The rotor then oscillates around a final resting position, the amplitude of the oscillations gradually decaying as the kinetic energy in the rotor is dissipated in friction and in eddy current losses induced by the field of the magnets in the stator and rotor. This process of coasting down and oscillating can take some time, particularly when the load torque diminishes with decreasing speed and the friction in the system is low. This characteristic can introduce some problems to the operation of the drive. For example, if the drive is switched off and an attempt is made to restart shortly afterwards, the rotor may still be in an oscillatory condition when the RPT signal indicates that the rotor is in an appropriate position for excitation to be reapplied to the windings. Under these conditions, the winding may be energized when the rotor has swung past its region of maximum developed torque towards the region of near zero torque (e.g. from point 3 in FIG. 2 towards point 4). At this point, the torque from the parking magnet opposes the (relatively small) main torque and the rotor is likely to stay in that position, failing to start.
In some known systems, the sensor is so positioned that it produces signal transitions in advance of the rotor reaching the aligned position. This is done for reasons associated with better operation at high speeds. It follows that, at zero or low speeds, where energization of the phase winding is linked directly to the edges of the sensor pulse train, these systems could allow energization of the winding in a region where the negative torque is sufficient to set the rotor into rotation in the opposite direction. While to overcome these and other problems it is possible simply to wait until the oscillations die away and the rotor is in a desired starting position, this may not be acceptable to a user requiring a rapid restart.